Nanofluids are stable mixtures of nanoscale particles dispersed in base fluids with good prospects in enhanced oil recovery in the petroleum industry. In this review, the mechanisms and evaluation methods of stable nanofluids were analyzed. The effects of nanoparticles on viscosity, electrical conductivity, and surface/interfacial tension of base fluids were discussed. The results of laboratory research and field tests revealed that nanofluids could improve the oil recovery through plugging and profile control, transformation of wettability of rock surface, changes in oil−water interface properties, and increases in the viscosity ratio between fluids. On the other hand, nanoparticles might stabilize oil−water emulsions produced during the extraction stage, adversely affecting subsequent oil−water separation processes, especially the electrical dehydration. However, careful analysis suggested lack of in-depth studies regarding the impacts of nanoparticles on droplet coalescence inside electro-dehydration plants. The present analyses will hopefully assist future investigations in nanofluidics.
Water droplets dispersed in crude oil have to be separated before processing and this is most commonly done by electrical dehydration. Under high strength electric fields, partial coalescence may occur. The critical electric field strength for partial coalescence occurance depends on several factors. In this paper, the effects of droplet diameter, conductivity, permittivity and other physical properties, such as viscosity, density and interfacial tension, on critical electric field strength have been studied experimentally, based on which a formula to predict the electric field strength is proposed. The critical electric field strength is shown to vary with the inverse of the square root of droplet radius, R-0.5. Its value is lower for droplets with high surfactant concentration. As the surfactant concentration increases, the slope Ecrit/ R-0.5 (k) decreases. The critical electric field strength increases as the concentration of alkali is increased. The slope k increases with the increase of conductivity and alkali concentration. On adding polymer, the critical electric field intensity is much higher than that of distilled water, and this suggests that droplets containing polymer do not * Viscosity ratio, Pa•s Interfacial tension, N•m-1 Induced charge, C Angle between the electric field direction and the droplet radius, ° Relative permittivity 0 Permittivity of free space F•m-1
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